1. Field of the Invention
The present invention relates to a carrier for an electrophotographic developer used for a two-component electrophotographic developer used in copiers, printers and the like and to an electrophotographic developer using the carrier, and relates particularly to a carrier for an electrophotographic developer comprising a resin-filled ferrite carrier which has a low true density and an elongated life, and is high in the charge imparting capability and stable, and to an electrophotographic developer using the carrier.
2. Description of the Related Art
The electrophotography is a method of developing by adhering toner particles in a developer to electrostatic latent images formed on a photoreceptor, and the developer used in this method is divided into a two-component developer composed of toner particles and carrier particles and a one-component developer composed of toner particles alone.
Among such developers, as the electrophotography using the two-component developer composed of toner particles and carrier particles, the cascade method used to be employed, but at present the magnetic brush method using a magnet roll is the mainstream.
In a two-component developer, the carrier particle is a carrier material which is mixed and stirred with the toner particle in a developing box filled with developer to impart a desired charge on the toner particle, further transports the charged toner to the surface of a photoreceptor to form toner images on the photoreceptor. The carrier particles remaining on a developing roll holding a magnet is again returned into the developing box from this developing roll, mixed and stirred with fresh toner particles, and repeatedly used in a certain period.
A two-component developer has, different from a one-component developer, functions of being mixed and stirred with toner particles, charging and further transporting the toner particles, and has a favorable controllability in designing a developer. Therefore, the two-component developer is suitable for full-color developing machines requiring high-quality images, high-speed printing machines requiring reliability and durability of image sustainability, and the like.
The two-component developer used in such a way needs to exhibit prescribed values from the initial stage in image characteristics such as image density, fogging in image, white spots, gradation and resolution, which also need not to vary in the continuous printing period and to be stably maintained. For stably maintaining these characteristics, the characteristics of the carrier particles contained in the two-component developer need to be stable.
As a carrier particle forming a two-component developer, an iron powder carrier such as an iron powder covered on its surface with an oxide film or an iron powder coated on its surface with a resin is conventionally used. Since such an iron powder carrier has a high magnetization and a high conductivity, it has an advantage of easily obtaining images good in the reproducibility of solid parts.
However, since such an iron powder carrier has a large true specific gravity of about 7.8 and too high a magnetization, stirring and mixing with toner particles in a developing box becomes liable to generate the fusion of the toner-constituting components onto the iron powder carrier surface, the so-called toner spent. Such a toner spent generation reduces the effective carrier surface area, and becomes liable to decrease the tribocharging capability with the toner particle.
In the resin-coated iron powder carrier, exfoliation of the surface resin due to stresses during endurance and exposure of the core (iron powder) high in conductivity and low in isolation breakdown voltage sometimes cause the charge leak. By such a charge leak, electrostatic latent images formed on a photoreceptor are broken; brush streaks and the like are generated on solid parts; and uniform images are hard to obtain. From these reasons, iron powder carriers such as an oxide-filmed iron powder and a resin-coated iron powder come not to be used at present.
As described in Patent Document 1 (Japanese Patent Laid-Open No. 59-48774), resin-coated ferrite carriers have been recently often used in which a ferrite core, which has a low true specific gravity of about 5.0 and also a low magnetization, is used in place of the iron powder carriers and the surface is coated with a resin, whereby the developer life has remarkably been elongated.
However, with evolution from an era of simple function copiers to that of combined machines with recently progressing office networking, and with the service systems shifting from a system where contracted servicemen periodically exchange developers, etc. for maintenance to a maintenance-free era, the market comes to have the larger demand for the more elongated life of the developers.
Further, offices come to accept full-color images and have the larger demand for high-quality images, and the toner particle size becomes smaller to obtain a high resolution.
Corresponding to this, the carrier particle size also has shifted to the small particle size having a high specific surface area with the need for quick charging of the toner in a desired charge. When the particle size distribution is totally made to be of small particle size, especially particles on the fine powder side become liable to bring about the phenomenon of the carrier particles scattering or adhering to a photoreceptor, the so-called carrier adhesion, and induce fatal image faults such as white spots. Therefore, the small particle size carrier must be controlled more narrowly in the particle size distribution width.
In such a situation, many magnetic powder-dispersed carriers in which fine magnetic microparticles are dispersed in resins are proposed in Patent Document 2 (Japanese Patent Laid-Open No. 05-40367), etc. to make the carrier particles light, and elongate the developer life.
Since such magnetic powder-dispersed carriers can be decreased in their true densities by reducing the magnetic microparticle amount, thus reducing the stress by stirring, the chipping and exfoliation of the coated film is prevented, thereby providing stable image characteristics for a long period.
However, since the magnetic powder-dispersed carriers are coated on the magnetic microparticles with binder resins, the carrier resistance is high. Therefore, they have a problem of having difficulty in providing a sufficient image density.
The magnetic powder-dispersed carriers sometimes raise such problems that magnetic microparticles are solidified with binder resins, and drop off by the stirring stress and the impact in developing machines, and that the carriers themselves break possibly due to their inferior mechanical strengths as compared with those of iron powder carriers and ferrite carriers conventionally used. Then, added dropwise-off magnetic microparticles and the broken carrier particles adhere to a photoreceptor, and sometimes cause image faults.
Additionally, the magnetic powder-dispersed carriers have a disadvantage of having a high residual magnetization and a high coercive force because of use of fine magnetic microparticles, and deteriorating the fluidity of the developers. Especially when magnet brushes are formed on a magnet roll, high-quality images are difficult to obtain because bristles of the brushes become hard due to the high residual magnetization and coercive force. Further, even when the carrier leaves the magnet roll, since the carrier magnetic aggregation does not come loose, and the carrier cannot be rapidly mixed with a supplied toner, the rising of the charge quantity is bad, there by raising a problem of causing image faults such as toner scattering and fogging in images.
Further, although the magnetic powder-dispersed carriers can be fabricated by two methods of the crushing one and the polymerization one, both have a problem of a high producing cost because the crushing method is bad in yield, and the polymerization method is complicated in the producing process.
Resin-filled type carriers in which voids in a porous carrier core are filled with a resin are proposed as one in place of the magnetic powder-dispersed carriers. For example, Patent Document 3 (Japanese Patent Laid-Open No. 11-295933) and Patent Document 4 (Japanese Patent Laid-Open No. 11-295935) describe a core or a hard magnetic core, polymers contained in the core pores, and carriers including coatings which coat the cores. These resin-filled type carriers are supposed to provide carriers which have little impact, desired fluidities, wide ranges of tribocharging values, desired conductivities, and certain ranges of the volume average particle sizes.
Here, Patent Document 3 supposes that as the core, various appropriate porous solid core carrier materials such as existing porous cores can be used. It describes that especially important are the porousness and the desired fluidity, and lists the soft magnetization, the porosity indicated by the BET area and the volume average particle size as noteworthy properties.
However, the porosity of about 1,600 cm2/g in BET area as described in an example of the Patent Document cannot provide a sufficiently low specific gravity even when filling with a resin, and cannot respond to the recently increasing need for the elongated life of developers.
Further, a sponge iron powder used in an example of the Patent Document cannot provide a sufficiently low specific gravity even when filling with a resin, and cannot at all come up to the desired elongated life.
Further, as described in the Patent Document, only by simply controlling the porosity expressed by the BET area, the specific gravity and mechanical strengths of a carrier after the resin filling are difficult to precisely control.
The measurement principle of BET area is to measure the physical adsorption and the chemical adsorption of a specific gas, and has no correlation with the core porosity. That is, even a core with pores scarcely present generally varies in the BET area depending on its particle size, particle size distribution, surface material quality, etc. Then, even if the porosity is controlled through the BET area measured in such a way, the core cannot be said to be one which can be filled fully with a resin. When a core with a high BET area but no or almost no pores is attempted to be filled with a large amount of a resin, the unfilled residual resin is present independently without adhering to the core, floats in the carrier, generates much aggregation of the particles, deteriorates the fluidity, largely varies charging characteristics on loosening of the aggregate in actual use periods, and so on, thereby making obtaining of stable characteristics difficult.
Additionally, only by controlling the BET area, a resin-filled carrier having three-dimensional laminate structures in which resin layers and ferrite layers are alternately present as in the present invention cannot be obtained.
Additionally, the Patent Document uses a porous core, and sets the total content of a resin with which the core is filled and a resin applied to the surface thereof to be preferably about 0.5 to about 10 wt % of the carrier. Further, in an example in the Patent Document, the resins do not amount to at most 6 wt % to the carrier. Such a small amount of the resins cannot achieve a desired low specific gravity, and can only provide performances similar to those of conventionally used resin-coated carriers.
Use of a hard magnetization core as described in Patent Document 4 brings about a disadvantage of worsening the fluidity of the developer because of its high residual magnetization and coercive force. Especially when magnet brushes are formed on a magnet roll, high-quality images are difficult to obtain because the bristles of the magnetic brushes become hard due to the high residual magnetization and coercive force. Further, even when the carrier leaves the magnet roll, since the carrier magnetic aggregation does not come loose, and the carrier cannot be rapidly mixed with a supplied toner, the rising of the charge quantity is bad, thereby raising a problem of causing image faults such as toner scattering and fogging in images.
Patent Document 5 (Japanese Patent Laid-Open No. 54-78137) describes a carrier for an electrostatic image developer in which pores and surface recesses of magnetic particles which have a lower bulk specific gravity than a substantially non-porous one and which are porous or have a large surface roughness are filled with a fine powder of an electric insulation resin, and contends that a developer is provided by using the carrier, which has advantages that, with the carrier having little toner accumulation on the carrier surface, the developer has stable powder characteristics and tribocharging characteristics under varying temperature and humidity conditions, and has a constant image density and no decrease in the image density with time lapse.
However, when pores of magnetic particles which are porous or have a large surface roughness are filled with a fine powder, if the iron powder is used as described in an example of the Patent Document, the pores are relatively easily filled with the fine powder, but is difficult to fill very fine voids with such a powder as is the case with voids in ferrite cores.
When trying to fill with a fine powder dispersed in a solvent, if the core is an iron powder, pores can be filled relatively uniformly as described above, but if the core is a ferrite one, the solvent only permeates voids of the core, thereby resulting in the dispersed fine powder present on the core surface. The powder easily drops off by being subjected to the mechanical stress in developing machines, leading to disadvantages of remarkable changes in the charging characteristics and resistance characteristics.
As described above, even the resin-filled carriers described in Patent Documents 3 to 5 cannot sufficiently satisfy the needs that the image density is sufficiently secured, and high-quality images can be maintained for a long period.
Especially, the resin-filled carriers disclosed in Patent Documents 3 to 5, which are obtained by filling with a fine powder of a resin or an electric insulation resin, are in their forms substantially no more than a carrier having an increased resin amount of a conventionally known carrier which is coated with a resin on its surface, and a carrier on such a level that the fine powder penetrates a few pores, and are not at a satisfiable level with respect to the charge imparting capability and its stability.
Accordingly, an object of the present invention is to provide a carrier for an electrophotographic developer which is used by mixing the carrier with a toner, sufficiently secures the image density and can maintain high-quality images for a long period, and an object to provide an electrophotographic developer using the carrier.